1. Field of the Invention
The present invention relates to a hybrid-type forklift, and more particularly, to a hybrid-type forklift having two power sources, that is, an engine and an electric motor.
2. Description of the Related Art
In general, a forklift known as an industrial vehicle uses an engine or an electric motor as a power source to drive a load for running operation (hereinafter, referred to as running load) and a load for performing a cargo handling operation (hereinafter, referred to as cargo handling load).
For example, JP 6-247190 A discloses an engine-type forklift for driving both the running load and the cargo handling load by means of a single engine. In performing the cargo handling operation while running, the operation of the engine is controlled based on an operation amount of a cargo handling lever, and the running speed of the forklift is controlled by means of a transmission mechanism having hydraulic clutches and brakes.
On the other hand, for example, JP 2003-192299 A discloses a battery-type forklift having two motors which cope with the running load and the cargo handling load, respectively. In this battery-type forklift, the motors are operated by being supplied with electric power from a battery so as to drive the running load and the cargo handling load.
Further, for example, JP 60-153436 A discloses a hybrid-type forklift that is mounted with an engine for charging a battery, in addition to two motors for driving the running load and the cargo handling load as disclosed in JP 2003-192299 A. In this hybrid-type forklift, an electric power generator is driven by the engine to charge the battery, and the motors operate by being supplied with electric power from the battery, to thereby drive the running load and the cargo handling load.
However, in the engine-type forklift as disclosed in JP 6-247190 A, during the cargo handling operation and the running operation, the operating condition of the engine is determined based on an operation amount of the cargo handling lever, and the running speed of the forklift is controlled by means of the transmission mechanism. As a result, it is difficult to realize an optimum operating condition for driving the engine at a low fuel consumption rate, with respect to torque patterns of both the cargo handling load and the running load. Also, since the hydraulic clutches and brakes are controlled to adjust the running speed of the forklift at this moment, a great energy loss is caused in some cases.
In general, a motor and a battery which are much larger in size than the engine are required when an attempt is made to obtain the same output as that of from the engine by using the motor. Accordingly, it is difficult to actually take out a large power when the running load and the cargo handling load are driven respectively by the motor and the battery mounted in the forklift as described in JP 2003-192299 A or JP 60-153436 A.